1. Field of the Invention
The present invention relates to a semiconductor device comprising a resistor-incorporating transistor.
2. Description of Related Art
FIG. 5 shows the arrangement of a conventional semiconductor device comprising a resistor-incorporating transistor, suitable for a digital circuit. More specifically, a P-type base area 92 is formed at the surface side of an N-type semiconductor substrate 91, and an N-type emitter area 93 is formed in this P-type base area 92. A base electrode 95 and an emitter electrode 96 are respectively connected to the base area 92 and the emitter area 93. The base electrode 95 and the emitter electrode 96 are formed, in a raised manner, on an insulating film 94 on the N-type semiconductor substrate 91 through respective contact holes 94a, 94b formed in the insulating film 94. A collector electrode (not shown) is bonded to an N+-type area 97 formed at the underside of the N-type semiconductor substrate 91.
A surface resistor 98 made of polysilicon for example is disposed on the insulating film 94. The surface resistor 98 is connected at its one end to the base electrode 95 raised from the insulating film 94. Using aluminium for example, a base pad 99 is formed on the surface of the other end of the surface resistor 98. The uppermost surface of this semiconductor device is covered with a surface protective film 90, and a portion of the surface of the base pad 99 is exposed to the outside through an opening 90a formed in the surface protective film 90.
A base wire (not shown) is bonded to that portion of the base pad 99 which is exposed through the opening 90a. Accordingly, a forward current entered from the base wire is given to the base electrode 95 through the surface resistor 98. The surface resistor 98 disposed on the insulating film 94 is better in resistance precision than a diffused resistor formed in the semiconductor substrate.
However, the arrangement above-mentioned involves the likelihood that when a forward surge voltage is applied across the base and the emitter, an overcurrent flows in the surface resistor 98 to generate heat, causing the same to be burnt. Further, the arrangement above-mentioned involves the likelihood that when a reverse surge voltage is applied across the base and the emitter, a leak current flows from the N-type semiconductor substrate (collector area) 91 to the base pad 99, generating a spot-like breakdown trace in the insulating film 94 under the base pad 99. Thus, the conventional semiconductor device comprising a resistor-incorporating transistor is not always sufficient in breakdown resistibility.
It is an object of the present invention to provide a semiconductor device having a resistor-incorporating transistor, which is improved in breakdown resistibility, and also to provide a method of producing such a semiconductor device.
According to the present invention, a semiconductor device comprises: a base area of the first conduction type formed on a semiconductor substrate; an emitter area of the second conduction type formed in the base area; a collector area of the second conduction type formed as joined to the base area; an impurity area of the first conduction type formed, as separated from the base area, in the collector area; a base electrode connected to the base area; a surface resistor connected, at a position thereof, to the base electrode; and a base pad connected to the surface resistor at other position thereof and also connected to the impurity area.
In other words, the semiconductor device of the present invention comprises a transistor having: a base area of the first conduction type formed on a semiconductor substrate; an emitter area of the second conduction type formed in the base area; a collector area of the second conduction type formed as joined to the base area; a base electrode connected to the base area; a surface resistor connected, at a position thereof, to the base electrode; and a base pad connected to the surface resistor at other position thereof, the base pad being connected to the collector area, and an impurity area of the first conduction type is formed, as separated from the base area, in the collector area at its portion joined to the base pad.
It is noted that the second conduction type is different from the first conduction type.
According to the arrangement above-mentioned, the collector area and the impurity area form a PN junction, thus forming a parasite Zener diode between the base pad and the collector area. Accordingly, even though a forward surge voltage is applied across the base and the emitter, a forward overcurrent entered into the base pad escapes to the collector area through the parasite Zener diode. This prevents an overcurrent from flowing in the surface resistor. This in turn prevents the surface resistor from being burnt due to the flow of an overcurrent therein, thus improving the breakdown resistibility against the forward surge voltage.
Further, when a reverse surge voltage not less than the breakdown voltage of the parasite Zener diode is applied across the base and the emitter, there occurs a breakdown phenomenon of the parasite Zener diode, causing a leak current to flow to the base pad from the collector area. Accordingly, for example when an insulating film is formed as covering the surface of the collector area, it is possible to prevent this insulating film from being broken and presenting a spot-like breakdown trace. This improves the breakdown resistibility against the reverse surge voltage, too.
Further, the parasite Zener diode is formed in the collector area at its portion connected to the base pad. Thus, there is no possibility of the semiconductor device being increased in size due to the provision of the parasite Zener diode.
These and other features, objects and advantages of the present invention will be more fully apparent from the following detailed description set forth below when taken in conjunction with the accompanying drawings.